Use the built in "video maker" app and create a new project with the video. You don't need to put in a theme or transitions or anything. Then export the video to a lower resolution. This will significantly reduce the video size.

Use the built in "video maker" app and create a new project with the video. You don't need to put in a theme or transitions or anything. Then export the video to a lower resolution. This will significantly reduce the video size.